Burdick, A. B.
J. W. Lesley in TGC 4:14 describes a situation which I feel pertains quite generally to plants grown from irradiated seed. Such plants must be a mosaic of various kinds of mutant and normal tissue.
My evidence on this is both direct and indirect. R1 seedlings of autodiploid L. pimpinelifolium almost always show evidence of sectoring in the first leaf. Sectores quite often appear in the second leaf also, and sometimes persist in the mature plant. Occasionally, a mutant sector includes the growing point so that an entire branch may appear mutant. As a rule, however, visible mutant sectors in R1 plants are lost or overgrown by normal appearing tissue. My indirect evidence is similar to Lesley's in that I sometimes recover a recessive mutation from one inflorescence on an R1 plant and not from another inflorescence on the same plant.
It is difficult to quantitatively appraise the extent and nature of this R1 sectoring in the "average" R1 plant but it seems safe to assume that many mutatios are produced in the 50 or more cells of the seed meristem by the exceedingly high radiation exposures we are using (equivalent to 30.000 - 50.000). The disapperience of visible mutant sectors from R1 plants as they mature indicates that there must be considerable competition among cell progenies as the plant matures and that those endowed with the greatest vigor will survive to be included in inflorescences and later branches.
This high exposure tolerance together with somatic selection for vigor providers an operational situation with which the improvement of tomato varieties by irradiation may be attempted. High dosage produces high mutation frequency (perhaps as high as 50% of recovered gamets from R1 plants. See note by A. B. Burdick, this report) while somatic selection provides a screen for mutations with undesirable dominant effects. This leaves, for recovery from the distal inflorescences, mutations with dominant beneficial effects, completely recessive recessives, and normal or unmutated types. Even if the frequency of "dominant beneficals" is extremely low in the seed, somatic selection should pick them up and include in the mature plant.
Even though one can find a highly mutable stage during meiosis, as W. R. Singleton has in corn, this is of little use in breeding work because such mutations must be recovered as single gametes and must be screened from among scores of R1 plants in a needle-in-a-haystack fashion. On the other hand, beneficial mutations induced in seed are screened "authomaticaly" and recovered in 50% of the R1 gametes.
We are of course, working on this problem, but we are not tomato breeders and our materials, for various reasons, are not ccmmercial varieties. Therefore, I would like to see someone with a specific commercial problem step in and let irradiation have a crack at it. I am quite willing to share our techniques and facilities with anyone who is interested.